1. Field of the Invention
The invention relates to a semiconductor light-emitting element. More particularly, it relates to a semiconductor light-emitting element which is made of a group III-V compound semiconductor or a mixed crystal thereof and has a hetero p-n junction, and produces a high output or high brightness and is especially suitable for photoexcitation, optical communications, etc.
2. Description of the Prior Art
Heretofore, a light emitting element made of a group III-V compound mixed-crystal system and exhibiting a high output has been manufactured by forming a p-n junction in such a way that an acceptor impurity such as Zn is introduced by diffusion into an n-type compound semiconductor substrate which has a wide band gap and which serves to transmit light. Alternatively, a p-n junction is formed in such a way that epitaxial layers which are respectively doped with an acceptor impurity such as Zn and a donor impurity such as Te are successively deposited on the substrate by liquid epitaxial growth.
In prior art semiconductor ligh-emitting elements, in general, the mixing ratio (band gap) of the p-type layer is substantially equal to or slightly greater than that of the n-type layer. It is therefore difficult to make the injection efficiency of carriers and, accordingly, the luminous efficiency, considerably high.
It is well known that the luminous efficiency can be enhanced by contouring the light emitting surface of the light emitting element into the shape of a dome. The luminous efficiency in the prior art in the case of such dome-shaped light-emitting element is about 5-10%.
Although the above-mentioned value of luminous efficiency of 5-10% is employable for ordinary uses, it is not suitable for a variety of light emitting devices for optical communications, etc. which have recently been proposed as light sources for optical communications, etc. and in which an optical fiber and a light emitting element are integrally formed. In order to couple a light emitting element with an optical fiber, the light emitting surface of the light emitting element should not be dome-shaped, but should be flat. However, when the light emitting surface is flat, the luminous efficiency is about one order of magnitude lower than that in the case of the dome-shaped element.
Accordingly, in order to form a light source device for optical communications by coupling a light emitting element with an optical fiber, it is necessary to sharply enhance the luminous efficiency of the light emitting element.
As a light emitting element of a group III-V compound mixed-crystal system which exhibits a high luminous efficiency, has a flat light emitting surface, and exhibits a high output, there has been proposed a light emitting element wherein, on an n-type compound semiconductor substrate, a mixed-crystal epitaxial layer which has the same conductivity type as that of the substrate and whose band gap is greater than that of the substrate is formed, and an acceptor impurity such as Zn is diffused from the surface of the epitaxial layer deeper than a hetero-junction which is formed between the substrate and the epitaxial layer to thereby form a p-n junction within the substrate and to provide a carrier confinement region between the hetero-junction and the p-n junction, thus achieving enhancement of the light output from the surface of the p-type layer (Japanese Patent Application Public-disclosure No. 159688/1975).
There has also been proposed a light emitting element wherein a p-type mixed-crystal layer is sandwiched between another p-type mixed-crystal layer and an n-type mixed-crystal layer which have a greater band gap than the first-mentioned p-type layer, to form a double hetero-junction, the p-type mixed-crystal layer of the smaller band gap being used as a carrier confinement region, thus achieving enhancement of the light output from the n-type mixed-crystal layer (Optics Communications, Vol. 4, No. 4, pp. 307-309, December, 1971).
Since, however, in both these light emitting elements the boundary of the carrier confinement region on the light emitting side is the hetero-junction, they have the serious disadvantage that light is reflected by the hetero-junction and that the luminous efficiency is lowered. It is also difficult to contour the light emitting surface into the shape of a dome.